This invention relates to vehicle brake systems, and more particularly to estimating brake pressure in the cylinder of a brake caliper in a brake apparatus.
Modern wheel driven vehicles often include sophisticated brake systems to improve handling of the vehicle and to enhance traction of the driving wheels. Traction control plays an important role in improving vehicle stability during acceleration. A typical traction control system (TCS) simultaneously controls both engine torque being applied to the driving wheels, and braking force applied to the drive wheels, to reduce spinning of the drive wheels during acceleration. In addition to reducing wheel spin, TCS is also used to preclude overheating of the brake system during braking while under the control of the TCS, or other brake control systems of the vehicle brake apparatus.
For the TCS to perform its desired function, however, it is important that the fluid pressure in the braking device be known to a significant degree of accuracy. For example, in a disk brake device having a caliper with a piston in a fluid cylinder for clamping brake pads against a rotor attached to the wheel, it is necessary that the fluid pressure in the cylinder be known and controlled for proper functioning of the brake during operation in a traction control mode, and also in other brake operational modes.
Although it is possible to install pressure sensors in the braking devices for directly measuring fluid pressure in the cylinder, this is seldom done, because to do so would increase cost, and increase the number of components to be calibrated. Every component that is added is also a potential point of failure, thereby reducing reliability of the brake system.
Instead of adding pressure sensors, most brake control systems utilize an algorithm for estimating fluid pressure in the cylinder. In the past, these algorithms have relied on empirically developed relationships between brake pressure and other parameters known to the brake system. These empirical relationships are determined through extensive testing of vehicles and brake systems, and are generally highly specific to a given brake system. Such relationships are costly to develop and implement in production, however, and do not provide the accuracy of information required for optimal operation of the brake system.
While it has been theoretically possible in the past to develop an algorithm for estimating fluid pressure in a brake cylinder from a mathematical model of a brake system, and then control the brake system in accordance with the model, the on-board computational capacity that would be required to handle all of the linear and non-linear parameters involved in running such an algorithm have been so prohibitive that brake system designers have heretofore opted instead to use the empirically developed relationships described above.
What is needed, therefore, is an improved method and apparatus for estimating fluid pressure in the cylinder of a brake caliper, on the basis of an actual mathematical model of the apparatus that can be used for controlling the brake apparatus in accordance with the estimated brake pressure.
Our invention provides such an improved method and apparatus for estimating brake pressure in a disk brake caliper, on the basis of a simplified mathematical model. The simplified model was developed through recognition by the inventors, that by limiting the model to operating conditions in which the caliper piston is holding the brake pads in contact with the rotor, higher order terms and certain fluid dependent terms of the model can be eliminated, thereby simplifying the model to the point that it can be readily used in an effective and efficient manner for estimating fluid pressure in the cylinder of the caliper, in an apparatus or method according to our invention. Our invention allows the pressure in the disk brake caliper to be accurately estimated without the use of sensors in the brake apparatus, and requires far fewer calibrations than prior approaches used to determine the pressure in the disk brake caliper.
Our invention may take the form of a method for estimating pressure in the cylinder of a brake apparatus having disk brake caliper including a cylinder adapted for exchanging fluid with a pressure source, and a piston movable in the cylinder in response to pressure within the cylinder from an exchange of fluid between the cylinder and the pressure source, where the piston is adapted for exerting force through a brake pad against a rotor of a disc brake. The method includes estimating a displacement of the piston while the piston is holding the brake pad in contact with the rotor, as a function of an exchange of fluid between the pressure source and the cylinder; and determining a pressure in the cylinder as a function of the displacement of the piston while the piston is holding the brake pad in contact with the rotor. The method may include estimating the exchange of fluid between the cylinder and pressure source as a function of a difference in pressure between the cylinder and the source.
Where the caliper has an effective spring rate, the displacement of the piston while the piston is holding the brake pad in contact with the rotor is a function of the effective spring rate.
Determining the pressure in the cylinder may include calculating the pressure from an equation defining pressure as a function of displacement. Determining the pressure may alternatively include selecting the pressure from a table of pressures as a function of displacement.
Our invention may also include operating the brake apparatus in accordance with the pressure in the cylinder, in a traction control system mode, or in some mode of operation other than TCS.
Our invention may also take the form of an apparatus or a computer program for carrying out a method according to our invention.
The foregoing and other features and advantages of our invention are apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.